Serpentine tube boiler



Aug. 22, 967 E- BERNSTE'N ETAL 3,336,974

SERPENTINE TUBE'BOILER 2 Sheets-Sheet l Filed May 5, 1965 ug. 22, i967E. BERNSTEIN ETAL 3,336,974

SERPENTINE TUBE BOILER Filed May 5, 1965 2 Sheets-Sheet 2 United StatesPatent flice 3,336,974 Patented Aug. 22, 1967 3,336,974 SERPENTINE TUBEBILER Ernest Bernstein, West Hartford, and David G. Randall,

United Aircraft Corpora- This invention relates in general to shell andtube heat transfer apparatus and more particularly to serpentine tubeboilers which are particularly adapted to the eflicient generation ofhigh temperature liquid metal vapor in a zero gravity environment.

In recent years considerable effort has been directed toward thedevelopment of a closed-circuit, Rankine-cycle, thermal power plant forthe long term generation of electrical power in outer space. In atypical power plant of this type an eflicient heat transfer fluid, suchas molten lithium, is circulated in a closed circuit, receiving thermalenergy during its passage through a nuclear reactor and releasing thisenergy to a second heat transfer fluid in a liquid metal boiler. Thesecond fluid, such as potassium, circulating in an independent secondclosed circuit extracts the heat from the lithium in the boiler whereinit is vaporized, the potassium vapor thus generated being utilized asthe driving force in electrical turbo-generating apparatus.

In conventional boiler design, the force of gravity is utilized toeffect a liquid-vapor separation in the boiler and to maintain theliquid being vaporized in intimate contact with the heated surfacestherein. In the absence of gravity, particularly in the environment ofouter space, a substitute force or forces must be provided to performthe functions normally effected by gravitational forces. For thisreason, in space power plant considerations, it has been necessary todepart somewhat from the traditional concepts of boiler design.

In addition to the zero gravity problem, it is quite obvious that aspace system boiler must be compact and lightweight because of launchinglimitations, reliable because of its inaccessibility after launching,durable for maximum utility and safety, and relatively stable inoperation. The problems inherent in such `a design are `furthermagnified because the system parameters usually specified requireoperation at elevated temperatures with working fluids of a verycorrosive nature contained in vessels constructed ofthe so-called exoticalloys.

It is an object of this invention to provide a boiler of theshell-and-tube type which is particularly adapted to the efficientgeneration of liquid metal Vapor in a zero gravity environment.

A further object of this invention is to provide a boiler in which thecentrifugal forces generated by a fluid flowing in an irregular path areutilized in lieu of gravity to effect liquid-vapor separation andmaintain the liquid being heated in intimate contact with the heatedsurfaces internal of the boiler for efficient heat transfer.

A still further object is to provide a boiler in which vaporization iseffected internal of a plurality of serpentine tubes which are adaptedto accommodate the differential thermal expansion of the boiler oversubstantially the entire tube length.

Still another object is the provision of a heat exchanger in which eachtube in the tube bundle is supported over substantially the entire tubelength to minimize the deleterious effects of severe accelerationloading and induced vibration.

An additional feature is the compact boiler characterized and ease ofconstruction.

provision of la lightweight, by high thermal efficiency Another featureis the provision of means whereby tube vibrations induced by two-phasefluid flow are effectively damped by the tube-locating and supportingspacers.

A further feature is the provision of a boiler which Will generate vaporof one hundred percent quality in a short developed length.

These and other objects and advantages will be evident or will bespecifically pointed out in connection with the following detaileddescription of the embodiment of the invention shown in the accompanyingdrawings.

FIGURE l is a fragmentary longitudinal view of one embodiment of thisinvention shown partly in section and with part of the shell brokenaway.

FIGURE 2 is a sectional View taken on line 2 2 of FIGURE l.

FIGURE 3 is a fragmentary longitudinal view of a representative boilertube shown partly in section, illustrating the serpentine configurationof the tube and the placement of the flashing orifice internal of thetube.

FIGURE 4 is a View taken on line 4-4 of FIGURE 3, illustrating the tubecurvature in the plane of the serpentine bends.

FIGURE 5 is an enlarged fragmentary View of a portion of the serpentinetube shown in FIGURE 3, illustrating particularly the flashing orifice.

As is most clearly illustrated in the preferred embodiment of theinvention shown in FIGURE l, the boiler may be seen to be generally ofthe shell--and-tube variety.

The fluid to be vaporized, typically a liquid metal such as potassium,enters the boiler through tube-side fluid inlet 10 and, after passingthrough the tubes 12, it is discharged from the boiler through tube-sidefluid outlet 14. It will be noted that the diameter of outlet 14 isgreater than the diameter of inlet 10 to accommodate the Volume increaseassociated with the conversion of the incoming liquid to vapor.

A shell-side fluid at high temperature, typically a liquid metal such aslithium heated by external means as in a nuclear reactor, is introducedto the boiler at shell inlet 16. The lithium passes through the tubebundle 18 around the outside of the individual tubes, surrendering heatin the process, and thence is discharged from the boiler through shelloutlet 20 for recirculation back to the heat source. It is, of course,obvious that, depending on the desired heat transfer conditions, theshell-side fluid may be circulated counter to the tube-side fluid, inwhich case the positions of the shell inlet and outlet will be reversed.

Direct impingem-ent of the hot lithium on the tube bundle is avoidedthrough the use of an impingement Ibaille 28 which is positioned in theinlet 16. Inlet tests conducted on a variety of heat exchangersdemonstrated that, in the absence of such baffling means in highperformance apparatus, severe vibrations could be promoted in theindividual tubes leading t-o tube fatigue and consequent tube failure,usually at the point where the tube enters the tube sheet. Baflle 28 isshown in its most preferred embodiment as of a general mushroom shape,smoothly contoured to minimize pressure drop and supported in the inlet16 by a plurality of radial legs 30. The fluid entering the shellthrough inlet 16 flows through the web formed by the legs 30, parallelto the stem 32, to the baffle crown 34 where it is distributed bothaxially and circumferentially with respect to the axis of the tubebundle. In this manner the impact for-ce of the incomf ing fluid streamis taken by the baille rather than by the several tubes adjacent theinlet, resulting in a more uniform temperature distribution, minimumtube vibration and reduced tube stress.

It will be understood that, at reduced power levels and lower lithiumflow rates, the baille may be dispensed with entirely and that,depending particularly on the shape -of the inlet, alternate bafiiedesigns are contemplated. In high performance equipment, however, whenthe shell side fiuid is to be introduced transverse to the tube axes,means for preventing the direct impingement of the fiuid stream on thetubes will be found advantageous.

In order to reduce the shell-side pressure drop it has also been found`advantageous to provide plenum chambers 24 and 25 adjacent the shellinlet and outlet respectively. For this purpose a pair of generallyspherical diffusers 22 and 26, forming extensions of the tubular shell36, are provided at the shell inlet and outlet, 16 and 20, respectively,to enhance the fiow distribution of the lithium in the cross-ow areas ofthe boiler. The provision of these diffusers promotes radial penetrationof the lithium into the interior of the tube bundle 18. Although it isnot shown in the accompanying drawings, it is contemplated that, in someinstances, an enlarged plenum chamber may be provided to permit fanningof the tubes in the cross-flow area to increase the tube-to-tube spacingand further reduce the shell-side pressure drop.

An inlet header or tube sheet 40 is connected to the inlet diffuser 22and a corresponding outlet header 42 is connected to the outlet diffuser26. As is illustrated in FIGURES 1 and 3, the individual tubes 12 in thetube bundle 18 extend through the respective headers at either end ofthe boiler and terminate at the outer surfaces of the headers. In thepreferred construction an annular seal weld is effected between the endof the tube and the surface of the tube sheet at 52, and the individualtubes are expanded into the tube sheet to form a metallurgical bondbetween the outer surface of the tube and the inner surface 50 of thehole in the tube sheet. The other end of the tube is similarly connectedto tube sheet 42.

Generally hernispherical closures 54 and 56 which contain and distributethe tube-side fluid to the headers are fixed to the headers 40 and 42,respectively.

Referring particularly to FIGURES l and 2, it may be seen that theindividual tubes 12 are substantially identical and are nested injuxtaposition in circumferential layers about the axis of the tubebundle to provide a compact boiler configuration and provide the maximumheat transfer surface area per unit volume.

As is most clearly illustrated in FIGURE 3, the individual tubes 12 areformed with a serpentine configuration 58 over the greater portion oftheir length. The mixture of liquid and vapor generated within the tubeson boiling of the liquid is effectively separated within the tube by thecentrifugal forces generated by the flowing fluid. Because of the tubeundulations, the liquid is maintained in intimate scrubbing contact withthe heated walls to permit additional vaporization of the liquid as aresult of local boiling.

In a gravity field, the liquid is generally effectively separated fromthe vapor by the force of gravity, although tube-side boiling in a unitof the type described is -enhanced by the additional liquid-vaporseparation produced by centrifugal force. In the absence of gravity,however, it is necessary to provide means for maintaining the liquid in-contact with the heated wall surface. Preferably, the liquid-Vapormixture is continually subjected to a change in ow direction during itstransit through the tubes and, to effect this continual change indirection, the tube is formed to present such a curved -path to themixture.

Even in a gravity field, tests have established that vapor of 100percent quality is more readily achieved in a unit of the type describedthan in conventional apparatus utilizing -straight tubes. This is truebecause at the higher vapor qualities, the effect of centrifugal forceas a separation means is many times greater than that of gravity,particularly at the higher fiow rates.

As might be anticipated in a high performance heat transfer unit, meansmust be provided to accommodate the differential thermal expansionbetween the tube bundle and the shell as the unit is brought to itsoperating temperature. The serpentine section of the tube also performsthis function. Moreover, the tube stress associated with bending as aresult of the differential expansion is accommodated over the entiresinuous length rather than being concentrated at a single bend, andexcessive localized stress in the tube wall is thereby avoided since thedefiections at each of the serpentine bends is small.

In the fabrication of a boiler for space applications, it may bepreferable to bend the tubes over their sinuous length from the plane ofthe sinuous bends to conform to the circular path of the tube layer inwhich the particular tube lies. This is illustrated in FIGURE 4. It is,of course, a means for increasing the compactness of the boiler with theanticipated weight-saving associated therewith. The radius of the bendthus imparted increases as the distance from the tube bundle axisincreases and, for this reason, the transverse bend in the outer tuberows may be dispensed With entirely since its efficacy decreases Withincreasing diameter.

In a preferred tube configuration, as is shown in FIG- URE 3, thesinuous section of the tube is preceded by a straight portion of shorterlength, constituting a fluid preheating area. Since the incoming fiuid,by proper design, remains a liquid in this area and the tube is filled,there is no need to provide a change in fiow direction in this area formaximum heat transfer efficiency.

Such a straight tube inlet portion serves an additional function. Itfacilitates the placement in the tube of a flashing orifice 70, shownmost clearly in FIGURES 3 and 5. The preferred orifice constructioncomprises an annular insert 72, formed from a metallurgical bondpromoting material, adjacent the inner wall of the tube and encircling asecond annular insert 74 over a substantial portion of its length. Theinsert 74 terminates at its upstream end in an enlarged internallythreaded end portion 75 which is adapted to receive a cooperatingthreaded orifice member 76. The insert 74 is fixedly held in position inthe tube by frictional forces or more preferably by the formation of ametallurgical bond established by swaging the tube 12 and, consequently,reducing its diameter, with the pertinent inserts in their correctposition. The insert 72 is utilized to promote formation of thismetallurgical bond in a manner well known in the art.

In the embodiment shown the orifice diameter 78 of member 76 is of thesame diameter as the passageway 80 in insert 74. However, member 76 isadapted to be insertable or removable after the tube bundle has beenassembled to permit utilization of different orifice members 76 withvarying orifice diameters to permit alteration of the tube-side pressuredrop in varying installations. With different fluids, flow conditionsand temperatures, preferred orifice diameters will change and, by thisconstruction such changes may be effected after assembly of the tubebundle.

In the tube-side vaporization of liquid metals, the interposition of aflashing orifice has been demonstrated to be necessary for stability ofoperation. Its purpose is to effect a sudden pressure drop within thetube causing at least part of the liquid to flash into vapor and, thus,to initiate vaporization at a predetermined location along the tubeaxis. For this reason the individual tube orifices are located in acommon plane perpendicular to the axis of the tube bundle.

It will, of course, be understood that in alternate boiler designs, theorifices may be located in different locations, as for example withinthe inlet header, and that the straight preheating tube section may thenbe dispensed with.

In high performance apparatus particularly, it is necessary to supportthe individual tubes within the tube bundle both from each other andfrom the shell. This is particularly true in apparatus designed forspace applications since the internally and externally inducedvibrations and acceleration loading could otherwise lead to prematuretube failure. In the construction shown most clearly in FIGURE 2, thepreferred tube spacing and tube positioning means comprises a pluralityof wave Washers 90 and annular rings 92 positioned between each tubelayer and further positioned at a plurality of locations axially of thetube bundle. Although for economic reasons the wave washers 90 andannular rings 92 are shown as separate elements, they may bepreassembled into a unitary structure, or machined as a single element,and still perform their supporting and spacing function. In theapparatus shown, such tube support means is provided axially along thetube bundle at regularly spaced locations corresponding to one-half ofthe wave pitch of .the tubes.

The wave washer 90 may be seen to be generally annular and in the formof a sine wave when viewed along the axis of the tube bundle. The wavepitch of the washer 90 conforms to the tube spacing and the individualtubes are received in the troughs formed between the individual wavecrests.

Annular rings 92 are provided between each tube layer, the spacingbetween adjacent layers corresponding to the combined thickness of thewave washer 90 and the annular ring 92. The rings 92 are adapted toconform closely to the outer periphery of the circumferential row oftubes which they encircle and further to conform closely at their outerperiphery to the inner circumference of the abutting wave washer. Theouter annular ring 96 is further adapted to closely conform to the innersurface of the tubular portion of the shell 36 to prevent bypass of theshell-side fluid between the shell and the tube bundle along surface 94.

The particular tube spacing means shown, comprising the combination ofthe annular rings 92 and the wave Washers 90, when positionedperiodically along the axis of the tube Ibundle serve to damp anyinduced vibrations generated in the tubes as a result of two-phase uidflow or boiling instability, or from external sources.

Further, boiler fabrication and assembly is considerably simplifiedthrough this spacer construction, since the tube bundle may becompletely assembled prior to incorporation into the shell and theheaders. The tube spacing means and particularly the wave washers serveto position the remaining tubes within the tube bundle after a radialline of tubes has been established.

In the actual assembly of the boiler, it is the usual practice to formthe straight portions of the sinuous tubes to a greater length than thatdesired in the finished product and, after installation and positioningof the inlet Y header 40 thereon, to cut the tubes off at the outersurface of the header. In this way, the concern with tube lengthtolerances is minimized since all tubes are cut flush with the headersurface at assembly.

Since the tubes 12 are arranged in annular rows in the tube bundle, anopen port is provided in the assembled tube bundle. To avoid ashort-circuiting of the shell-side fluid through this port, aflow-defining insert 100 is provided along the axis of the tube bundle.For weight-saving considerations, insert 100 is preferably in the formof a tube, which is mounted at each end to the abutting header. Toaccommodate the differential thermal expansion between the tube `bundleand the shell, the insert is shown slidably mounted to the inlet header40. For this purpose, a generally cylindrical pin 102 is ixedly mountedin the end of insert 40 so that a substantial portion projects beyondthe end of the insert, the projecting portion of the pin riding in ablind hole 104 in the lower surface of header 40 after assembly of theboiler.

. In this construction, a boiler has been provided which willefliciently vaporize liquid metals, or other heat transfer liquids, in azero gravity environment in a relatively simple and lightweightconstruction. Further, the boiler of this invention will be relativelyimmune to the deleterious effect of internallyv and externally producedvibrations and to acceleration loading. Further, through the interchangeof orifices, substantially identical boiler apparatus may be easilyadapted to accommodate changes in liow and temperature conditions anddifferent fluids. Still further, vapor of percent quality may be readilygenerated in tubes of very short developed length.

While a preferred embodiment of the invention has been shown anddescribed, it will be understood that various changes and modificationsmay "be made thereto without departing from the spirit of the inventionas defined in the following claims.

We claim:

1. In a liquid metal boiler of the shell and tube type, a generallycylindrical tube bundle comprising:

a plurality of substantially identical tubes nested in juxtaposition incircumferential layers about the axis of the tube bundle, each of saidtubes having a serpentine configuration over a major portion of itslength,

means for supporting said tubes in radial and circumferential spacedrelation about the axis of the tube bundle, and

a flashing orifice internal of each tube intermediate its length, eachorifice being of equal size and positioned longitudinally of the tubebundle in substantial correspondence with each other orifice.

2. The tube bundle of claim 1 wherein:

each of said tubes is shaped over its serpentine length to conform tothe circular path of the tube layer in which it is positioned.

3. The tube bundle of claim 2 wherein the tube supporting meanscomprises:

a plurality of circumferential wave washers positioned adjacent eachtube layer, the wave pitch corresponding to the circumferential tubespacing, the washers being adapted to retain the tubes between ladjacentwaves, and

a plurality of annular rings between each tube layer,

the thickness of the annular rings andthe wave washers defining thespacing between adjacent tube layers.

4. The tube bundle of claim 3 wherein:

the tubes are supported at spaced locations axially of the tube bundle.

5. The tube bundle of claim 3 wherein:

the tube supporting means are positioned axially of the tube bundle atregularly spaced locations over substantially the entire serpentineportion of the tubes, the spacing corresponding to one-half of the tubewave pitch.

6. A liquid metal boiler especially adapted to the genleration of vaporin a zero gravity environment comprismg:

a tubular shell having an inlet and an outlet,

a tube inlet header joined to said shell at one end,

a tube outlet header joined to said shell .at its other end forming achamber therebetween,

a generally cylindrical tube bundle extending through said chamber andaffixed to said headers, the tube bundle comprisinga plurality ofsubstantially identical sinuous tubes nested in juxtaposition in circum-:ferential layers about the axis of the tube bundle,

a flashing orifice positioned in each tube adjacent the inlet header,each orifice substantially corresponding in internal diameter to eachother orifice,

means for supporting said tubes in spaced relation about the axis of thetube bundle, and

means disposed in the shell inlet for preventing direct impingment ofthe shell-side uid on the tube bundle.

7. The `boiler of claim 6 in which:

each of said tubes comprises a straight inlet section of substantiallength and a sinuous section of greater length, and

each orifice is positioned in said straight tube section.

eration of vapor in a zero gravity environment comprisa tubular shell,

a pair of diffusers attached to said shell, one at each end of saidshell, each diffuser having a fluid connection thereto,

a header attached to each diffuser at its outer end forming a chambertherebetween,

a generally cylindrical tube bundle extending longitudinally throughsaid chamber and affixed to said headers, the tube bundle comprising aplurality of tubes nested adjacent one another in annular rows about theaxis of the tube bundle, each of said tubes having a straight portion ofsubstantial length and a sinuous portion of greater length, each of saidtubes being of constant cross-section throughout its length.

a ashing orice positioned in the flow path of each tube in the straightportion thereof, at least part of 30 said orifice being removable afterassembly of the tube bundle,

a ow-dening insert positioned along the centerline of said tube bundleand affixed to each of said headers, said insert being slidably mountedto at least one of said headers,

means for supporting said tubes in spaced relation about the axis of thetube bundle, and

a ba'le disposed in the liuid connection of one of said sphericaldiffusers for preventing direct impingement of the shell-side lluid onthe tube bundle.

L1. The boiler of claim 10 wherein:

the individual orifices are substantially identical and are positionedin a common plane perpendicular to the axis of the tube bundle.

12. The boiler of claim 11 wherein:

the tube supporting means are positioned at regularly spaced locationsaxially of the tube bundle along the sinuous portion of the tubes.

References Cited UNITED STATES PATENTS 1,525,094 2/1925 Jones 165-1611,748,121 2/1930 Gay 165--174 1,815,932 7/1931 Sieder 165-161 2,519,0848/1950 Tull 165-163 3,118,497 1/1964 Olson 165-159- 3,134,432 5/1964Means 165-161 ROBERT A. OLEARY, Primary Examiner.

CHARLES SUKALO, Examiner.

1. IN A LIQUID METAL BOILER OF THE SHELL AND TUBE TYPE, A GENERALLYCYLINDRICAL TUBE BUNDLE COMPRISING: A PLURALITY OF SUBSTANTIALLYIDENTICAL TUBES NESTED IN JUXTAPOSITION IN CIRCUMFERENTIAL LAYERS ABOUTTHE AXIS OF THE TUBE BUNDLE, EACH OF SAID TUBES HAVING A SERPENTINECONFIGURATION OVER A MAJOR PORTION OF ITS LENGTH, MEANS FOR SUPPORTINGSAID TUBES IN RADIAL AND CIRCUMFERENTIAL SPACED RELATION ABOUT THE AXISOF THE TUBE BUNDLE, AND A FLASHING ORIFICE INTERNAL OF EACH TUBEINTERMEDIATE ITS LENGTH, EACH ORIFICE BEING OF EQUAL SIZE AND POSITIONEDLONGITUDINALLY OF THE TUBE BUNDLE IN SUBSTANTIAL CORRESPONDENCE WITHEACH OTHER ORIFICE.